Integrated gearbox/encoder and control system

ABSTRACT

An integrated gearbox/encoder and control system that includes: a gearbox with an output shaft connected to a mechanical load; a first sensor detecting the rotary position of the output shaft; a motor; a second sensor detecting the rotary position of the motor; and a system controller controlling motive drive to the motor. The two rotary position sensors permit direct determination of gearbox backlash which can be used in motor control. A drive current sensor similarly permits determination of a vibration signature for comparison with a standard.

CLAIM OF PRIORITY

This application claims priority under 35 U.S.C. 119(e)(1) to U.S.Provisional Application No. 61/139,765 filed Dec. 22, 2008.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to motor control systems and particularlyan integrated gearbox/encoder and control system.

BACKGROUND OF THE INVENTION

It is common practice to combine electric motors and gear reductionsystems to reduce system speed to a desired operating point. All gearsystems have an internal loss of rotational motion called backlash.Errors created by system backlash make it impossible for a controller toaccurately position a load connected to the gearbox if the only sourceof position information is attached to the motor. A common solution addsan additional position feedback device on the load side as the masterposition reference. This configuration of motor and sensors is widelyused, it does not take full advantage of the information available toimprove the control system performance.

SUMMARY OF THE INVENTION

This invention is an integrated gearbox/encoder and control system thatincludes: a gearbox with a first output shaft that couples to amechanical load; a first integrated sensor that determines the positionof a first output shaft; a motor with a second output shaft; a secondsensor that determines the rotary position of the second output; and asystem controller coupled to motor drive electronics and the firstsecond sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 is a schematic diagram of the invention;

FIG. 2 illustrates the dynamic compensation provided by the disclosedinvention;

FIG. 3 illustrates a flow chart of an embodiment of the use of measuredbacklash of this invention;

FIG. 4 illustrates a flow chart showing another use of the measuredbacklash of this invention;

FIG. 5 illustrates an example of a prior art control sequence commandedby a controller;

FIG. 6 illustrates a flow chart of an embodiment of the inventionincluding a second control loop around two feedback devices;

FIG. 7 illustrates a flow chart of an embodiment of the use of ameasured vibration profile in this invention; and

FIG. 8 illustrates flow chart 800 of an embodiment of the use ofmeasured gearbox flexure/deflection in this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is an integrated gearbox/encoder and control system. Thisapplication describes numerous specific details in order to provide athorough understanding of the present invention. One skilled in the artwill appreciate that one may practice the present invention withoutthese specific details. Additionally, this disclosure does not describesome well known items in detail to not obscure the present invention.

FIG. 1 illustrates the parts of this invention. Gearbox or jack screw 1has an output shaft coupled to mechanical load 2. FIG. 1 illustrates noparticular geometry for gearbox 1. The position of mechanical load 2 isto be controlled. Sensor 3 is integrated into gearbox 1. Sensor 3 may bemagnetic, optical or based on strain. Sensor determines the rotaryposition of the output shaft of gearbox 1. Motor 4 has a rotary positionsensor 5 rigidly coupled to the shaft, a controller 6 and motor drivepower electronics 7 commanded by controller 6. Motor 4 may be electric,pneumatic or hydraulic. Rotary position sensor 5 may be may be magnetic,optical or based on strain. Controller 6 is preferably a proportional,integral and derivative controller. Current sensor 8 measures the drivecurrent from motor drive electronics 7 to motor 4.

In any similarly configured system the two position sensors providingposition information leads to several benefits permitting the creationof new control software. Such new control software would improveperformance by incorporating information into the control system modeland making adjustments to the velocity, position or toque values.

On startup controller 6 can automatically advance and reverse the motorat slow speed and low torque just enough to cause engagement of thegears in either direction. Controller 6 compares the motion of the motormeasured by rotary position sensor 5 to the motion of the load measuredby sensor 3. Controller 6 can thus measure the actual mechanicalbacklash and save this measured value. Controller 6 may periodicallymeasure this backlash. An increase in the measured value of the backlashover time indicates wear in the system. Controller 6 may be programmedwith a limit for the wear as a criteria for repair or replacement.

FIG. 2 illustrates a flow chart example of this backlash determination.Flow chart 200 begins with start block 201. In the preferred embodimentcontroller 6 makes a backlash determination upon each initialapplication of electric power, start up. It is also possible toperiodically make this backlash determination following start up. Flowchart 200 advances motor 4 at low speed and low torque as commanded byblock 202.

Test block 203 determines if motion is detected in load sensor 3. If nomotion is detected in load sensor 3 (No at test block 203), then testblock 203 repeats. This repeated test takes place while controller 6continues to control motor 4 as commanded by block 202. If motion isdetected in load sensor 3 (Yes at test block 203), then block 204 storesthe detected motor position M1 and the detected load position L1 andreverses the drive to motor 4.

Test block 205 determines if motion is detected in load sensor 3. If nomotion is detected in load sensor 3 (No at test block 205), then testblock 205 repeats. This repeated test takes place while controller 6continues to control motor 4 in reverse as commanded by block 204. Ifmotion is detected in load sensor 3 (Yes at test block 205), then block206 stores the detected motor position M2 and the detected load positionL2 and stops the drive to motor 4.

Block 207 calculates the backlash. This backlash calculation is basedupon the difference in the change in the motor sensor 5 detectedpositions and the change in the load sensor 3 detected position. Thusthe backlash BL is give by:

$\begin{matrix}{{BL} = {{\Delta\; M} - {\Delta\; L}}} \\{= {\left( {{M\; 2} - {M\; 1}} \right) - \left( {{L\; 2} - {L\; 1}} \right)}}\end{matrix}$The control of flow chart 200 continues via continue block 208. Thisdetermined backlash can be stored and used in later control as outlinedbelow.

FIG. 3 illustrates flow chart 300 of an embodiment of the use ofmeasured backlash in this invention. Flow chart 300 begins with startblock 301. Block 302 performs a backlash measurement. This backlashmeasurement could be made as illustrated in FIG. 2. As previously noted,this backlash measurement could be preformed upon each initialapplication of electric power to controller 6 or at periodic intervalsduring system operation.

Test block 302 compares the current measured backlash cBL with a priorstored initial backlash iBL. Test block 302 determines whether thecurrent backlash cBL is less than or equal to the sum of the initialbacklash iBL and an empirically determined constant α. If this is true(Yes at test block 303), flow chart 300 continues at continue block 304.If this is not true (No at text block 303), then block 305 flags aremedial operation such as repair or replacement. As previously noted,such an increase in the current measured backlash above an initialbacklash indicates wear in the drive system indicating a need ofremedial action.

FIG. 4 illustrates flow chart 400 showing another use of the measuredbacklash. Flow chart 400 begins at start block 401. Test block 402determines whether controller 6 orders a motor direction reverse. Thistest takes place while controller 6 continues to control motor 4. Ifthere is no direction reverse (No at text block 402), then flow chart400 continues at continue block 403. If controller 6 orders a motordirection reversal (Yes at text block 403), then block 404 alters thecontrol program. Block 404 adds the current measured backlash to theposition term in the control program of controller 6. This addition ofthe backlash amount accounts for the amount of rotation of motor 4before motion at the output of gearbox 1. This alternation of thecontrol program of controller 6 permits controlled operation through thebacklash of the motor reversal.

In any motion system there are inflection points where the accelerationforce changes from a non-zero value to a zero value. FIG. 5 illustratesan example of a prior art control sequence commanded by controller 6.FIG. 5 illustrates a velocity profile over time. Region 501 is beforethe motor is controlled. During region 501 the position is constant andboth the velocity and acceleration are 0. Region 502 is an accelerationregion. During region 502 the load position changes. The load velocityis increasing and the acceleration is greater than 0. Region 502represents an initial acceleration of the load. FIG. 5 illustrates aconstant acceleration by the constant velocity slope. Such a constantacceleration is not required. The load acceleration may vary duringregion 502. Region 503 represents a constant velocity region. The loadposition changes continually, the load velocity is constant and the loadacceleration is 0. Region 504 is a deceleration region. During region504 the load position changes. The load velocity is decreasing and theacceleration is less than 0. Region 504 represents deceleration of theload. FIG. 5 illustrates a constant deceleration by the constantvelocity slope. Such a constant deceleration is not required. The loaddeceleration may vary during region 504. Region 505 is after thecontrolled operation. In region 505 the position is constant and thevelocity and acceleration are 0.

FIG. 5 illustrates several inflection points of changing acceleration.The first of these is at point 511 where the acceleration changes from 0to a non-zero positive value. When the load has been accelerated up to aconstant target speed, the acceleration then changes from non-zero to 0with the load running at the desired speed. This occurs at point 512. Atpoint 513 the acceleration changes from 0 to a non-zero negative value.Finally at point 514 the acceleration changes from a negative value to0. Following point 514 the load is a rest at a constant position withboth the velocity and acceleration 0.

At these inflection points 511, 512, 513 and 514 gearbox 1 becomesunloaded. Normally there is some instability in the control systemusually resulting in momentary overshoot. Generally there is a highimpact load called jerk that can damage gear teeth as the gears returnto a meshed state. Motor control can be improved by taking into accountthe dynamic property of the backlash by adjusting the torque and speedloops to smoothly engage the gears as the system transitions throughinflection points, thereby eliminating stress.

In the displacement profile illustrated in FIG. 5 the inflection pointsare easy to identify. When the system is at rest, zero velocity, it isimpossible to determine the engagement of the gears. As the motorstarts, backlash is immediately taken up by the rotation of the motorcausing jerk or impact stress as the gears begin to move. When thesystem stops accelerating, and tries to achieve constant speed, anotherinflection point occurs during which the load can overrun slightly andthe gears become momentarily disengaged. In this way, every timeacceleration changes from a zero to a non-zero state, inflection pointsoccur which present abnormal stresses to the gears and which cannot beadequately compensated in the control system.

FIG. 6 illustrates flow chart 600 of a proposed embodiment of theinvention including a second control loop around the two feedbackdevices. This permits precise regulation of the stress created by thebacklash in its various states. Flow chart 600 begins with start block601. Test block 602 determines if the control profile is near points 511or 513. Each of these control points are where the acceleration changesfrom zero to non-zero. If this is true (Yes at test block 602), thencontrol switches from the programmed command profile such as illustratedin FIG. 5 to a second control loop. Block 603 controls motor 4 at a lowacceleration until the position change detected by motor sensor 5 movesan amount corresponding to the previously measured backlash BL past theposition change detected by load sensor 3. Block 603 exits the secondcontrol loop and returns to the initial command profile upon reachingthis point. Reaching this point assures that the backlash is wound outof gearbox 1. Command control can proceed with the assurance that therewill be a minimal jerk within gearbox 1.

Whether test block 602 did not detect proximity to points 511 or 513, orblock 603 completes, test block 604 determines if the control profile isnear points 512 or 514. Each of these control points are where theacceleration changes from non-zero to zero. If this is true (Yes at testblock 604), then control switches from the programmed command profilesuch as illustrated in FIG. 5 to a second control loop. Block 604controls motor 4 to reduce the acceleration upon nearing the inflectionpoint 512 or 514. The interval before the inflection point foracceleration reduction depends on the amount of backlash BL. A large BLrequires a large offset from the inflection point. A small BL requires asmaller offset. This torque reduction reduces or eliminates the torqueoverrun upon reaching a steady state condition reducing the jerk uponremeshing the gears. Whether test block 604 did not detect proximity topoints 512 or 514, or block 604 completes, control continues with themain command profile at continue block 606.

Normal torque control in a motor drive involves providing sufficientcurrent to allow the motor to turn the attached load. Since most motorcontrol systems involve monitoring of current the motor current isreadily available to the control system. FIG. 1 illustrates currentsensor 8 as part of motor drive electronics 7. Mechanical loads thathave eccentric properties, such as cams or rotary knives, requirecurrent compensation to maintain commanded speed.

By precisely measuring the torque loop waveforms via current sensor 8 itis possible to measure vibration signatures in the gearbox. Subtlevibration in the gears are commonly described in manufacturer'sspecifications as torque ripple. Since the torque ripple of a gearsystem is a function of the mechanical construction details, a signaturewaveform will appear as noise in the torque loop of the control system.

If the vibration levels exceed some determined normal operating range,it would indicate an impending failure in the mechanism. This out oftolerance condition can be reported through the control system and actedupon as a preventive maintenance event instead of a catastrophicfailure.

FIG. 7 illustrates flow chart 700 of an embodiment of the use ofmeasured vibration profile in this invention. Flow chart 700 begins withstart block 701. Block 702 performs a torque profile measurement.

Test block 703 compares the current measured torque profile with a priorstored initial torque profile. Test block 703 determines whether thetorque profile is within predetermined tolerances of the prior torqueprofile. If this is true (Yes at test block 703), flow chart 700continues at continue block 704. If this is not true (No at test block703), then block 705 flags a remedial operation such as repair orreplacement. As previously noted, this out of tolerance torque profileindicates an impending failure.

Gear reducers and other rolling mechanisms have a certain deflectioncharacteristic called torsion which is intrinsic to the materials usedin their design. Over time, repeated cycling will load the materials andthey will fatigue. This fatigue is a unique material property whichsubstantially reduces the deflection of the parts. By comparing themeasured flexure of the system and observing the change in thedeflection of the gear train, a threshold of performance can beestablished that indicates that catastrophic failure may be imminent.Thus the measurement of deflection by the control system comparing themotor position with the gear system output position leads to a new andnovel means of preventing fatigue failure in the gear components.

FIG. 8 illustrates flow chart 800 of an embodiment of the use ofmeasured gearbox flexure/deflection in this invention. Flow chart 800begins with start block 801. Block 802 performs a flexure/deflectionmeasurement.

Test block 803 compares the current measured flexure/deflection with aprior stored initial flexure/deflection. Test block 803 determineswhether the flexure/deflection is within predetermined tolerances of theprior flexure/deflection. If this is true (Yes at test block 803), flowchart 800 continues at continue block 804. If this is not true (No attext block 803), then block 805 flags a remedial operation such asrepair or replacement. As previously noted, this out of toleranceflexure/deflection indicates an impending failure.

1. An integrated gearbox/encoder and control system, comprising: agearbox with a first output shaft that couples to a mechanical load; afirst sensor integrated into said gearbox, said first sensor detecting arotary position of said first output shaft of said gearbox; a motor witha second output shaft that couples to said gearbox; motor driveelectronics connected to said motor controlling motive power supplied tosaid motor to control motion of said motor; a second sensor coupled tosaid second output shaft of said motor detecting rotary position of saidsecond output shaft of said motor; and a system controller connected tosaid motor drive electronics, said first sensor and said second sensor,said system controller controlling motion of said motor through saidmotor drive electronics, said system controller operable to measure agearbox backlash by controlling said motor to advance at a low velocityand a low acceleration until detecting motion at said first sensor,thereafter storing a first detected rotary position of said first sensorand a first detected rotary position of said second sensor, controllingsaid motor to reverse direction and advance at said low velocity andsaid low acceleration until detecting motion at said first sensor,thereafter storing a second detected rotary position of said firstsensor and a second detected rotary position of said second sensor, andcalculating a backlash from a difference between a difference betweensaid second and first detected rotary positions of said second sensorand a difference between said second and first detected rotary positionsof said first sensor.
 2. The integrated gearbox/encoder and controlsystem of claim 1, wherein: said system controller is operable toperiodically measure said gearbox backlash.
 3. The integratedgearbox/encoder and control system of claim 1, wherein: said systemcontroller is operable to measure said gearbox backlash once eachinitial application of electric power.
 4. The integrated gearbox/encoderand control system of claim 1, wherein: said system controller isfurther operable to compare said measured gearbox backlash to astandard, continue system controller operation upon a first result ofsaid comparison, and abort system controller operation and requestremedial action upon a second result of said comparison opposite to saidfirst result.
 5. The integrated gearbox/encoder and control system ofclaim 4, wherein: said system controller is operable to compare saidmeasured gearbox backlash to a sum of an initial measured backlash andan empirically determined constant, said first result of said comparisonis said backlash less than or equal to said sum, and said second resultof said comparison is said backlash greater than said sum.
 6. Theintegrated gearbox/encoder and control system of claim 1, wherein: saidsystem controller is further operable to detect commanded directionreversal of said motor, and upon detection of a commanded directionreversal of said motor add said measured gearbox backlash to acalculated position control term in controlling motion of said motor. 7.The integrated gearbox/encoder and control system of claim 1, wherein:said system controller is further operable to detect a commanded changein control of said motor from zero acceleration to non-zeroacceleration, and upon detection of said commanded change in control ofsaid motor from zero acceleration to non-zero acceleration, command saidmotor at low acceleration until said second sensor indicates an movementcorresponding to said measured gearbox backlash greater than said firstsensor indicates movement.
 8. The integrated gearbox/encoder and controlsystem of claim 1, wherein: said system controller is further operableto detect commanded change in control of said motor from non-zeroacceleration to zero acceleration, and upon detection of said commandedchange in control of said motor from non-zero acceleration to zeroacceleration, command said motor at reduced acceleration for an amountbefore said commanded change in control of said motor from non-zeroacceleration to zero acceleration corresponding to said measured gearboxbacklash.
 9. A integrated gearbox/encoder and control system comprising:a gearbox with a first output shaft that couples to a mechanical load; afirst sensor integrated into said gearbox, said first sensor detecting arotary position of said first output shaft of said gearbox; a motor witha second output shaft that couples to said gearbox; motor driveelectronics connected to said motor controlling motive power supplied tosaid motor to control motion of said motor; a second sensor coupled tosaid second output shaft of said motor detecting rotary position of saidsecond output shaft of said motor; a current sensor connected to saidmotor drive electronics detecting drive current from said motor driveelectronics to said motor; and a system controller connected to saidmotor drive electronics, said first sensor and said second sensor, saidsystem controller controlling motion of said motor through said motordrive electronics, said system controller operable to determine avibration signature for said motor from said drive current, compare saidvibration signature to a standard, continue system controller operationupon a first result of said comparison, and abort system controlleroperation and request remedial action upon a second result of saidcomparison opposite to said first result.
 10. A integratedgearbox/encoder and control system comprising: a gearbox with a firstoutput shaft that couples to a mechanical load; a first sensorintegrated into said gearbox, said first sensor detecting a rotaryposition of said first output shaft of said gearbox; a motor with asecond output shaft that couples to said gearbox; motor driveelectronics connected to said motor controlling motive power supplied tosaid motor to control motion of said motor; a second sensor coupled tosaid second output shaft of said motor detecting rotary position of saidsecond output shaft of said motor; and a system controller connected tosaid motor drive electronics, said first sensor and said second sensor,said system controller controlling motion of said motor through saidmotor drive electronics, said system controller operable to determine adeflection characteristic for said motor from said first sensor and saidsecond sensor, compare said deflection characteristic to a standard,continue system controller operation upon a first result of saidcomparison, and abort system controller operation and request remedialaction upon a second result of said comparison opposite to said firstresult.
 11. A method of motor control comprising the steps of: detectinga rotary position of an output shaft of a gearbox connected to a load;driving an input shaft of the gearbox with a motor; supplying motivepower to the motor; detecting rotary position of the input shaft of thegearbox; controlling the motive power supplied to the motor inconjunction with the detected rotary positions of the gearbox inputshaft and output shaft to control position of the load; and measuring agearbox backlash by controlling said motor to advance at a low velocityand a low acceleration until detecting motion at said first sensor,thereafter storing a first detected rotary position of said first sensorand a first detected rotary position of said second sensor, controllingsaid motor to reverse direction and advance at said low velocity andsaid low acceleration until detecting motion at said first sensor,thereafter storing a second detected rotary position of said firstsensor and a second detected rotary position of said second sensor, andcalculating a backlash from a difference between a difference betweensaid second and first detected rotary positions of said second sensorand a difference between said second and first detected rotary positionsof said first sensor.
 12. The method of claim 11, wherein: said step ofmeasuring a gearbox backlash includes periodically measuring the gearboxbacklash.
 13. The method of claim 12, wherein: said step of periodicallymeasuring the gearbox backlash includes measuring the gearbox backlashonce each initiation of the method.
 14. The method of claim 11, furthercomprising the steps of: comparing the measured gearbox backlash to astandard; continuing operation upon a first result of the comparison;and aborting operation and requesting remedial action upon a secondresult of the comparison opposite to the first result.
 15. Theintegrated gearbox/encoder and control system of claim 14, wherein: saidstep of comparing the measured gearbox backlash to a standard comparesthe measured gearbox backlash to a sum of an initial measured backlashand an empirically determined constant; said first result of saidcomparison is the backlash less than or equal to said sum, and saidsecond result of said comparison is the backlash greater than said sum.16. The method of claim 11, further comprising the steps of: detecting acommanded direction reversal of the motor; and upon detection of acommanded direction reversal of the motor adding the measured gearboxbacklash to a calculated position control term in controlling motion ofthe load.
 17. The method of claim 11, further comprising the steps of:detecting a commanded change in control of the motor from zeroacceleration to non-zero acceleration; and upon detection of thecommanded change in control of the motor from zero acceleration tonon-zero acceleration, commanding the motor at low acceleration untilthe detected rotary position of the input shaft of the gearbox indicatesa movement corresponding to said measured gearbox backlash greater thanthe detected rotary of the output shaft of the gearbox indicatesmovement.
 18. The method of claim 11, further comprising the steps of:detecting a commanded change in control of the motor from non-zeroacceleration to zero acceleration; and upon detection of a commandedchange in control of the motor from non-zero acceleration to zeroacceleration, commanding the motor at reduced acceleration for an amountbefore the commanded change in control of the motor from non-zeroacceleration to zero acceleration corresponding to the measured gearboxbacklash.
 19. A method of motor control comprising the steps of:detecting a rotary position of an output shaft of a gearbox connected toa load; driving an input shaft of the gearbox with a motor; supplyingmotive power to the motor; detecting rotary position of the input shaftof the gearbox; controlling the motive power supplied to the motor inconjunction with the detected rotary positions of the gearbox inputshaft and output shaft to control position of the load; detecting drivecurrent of the motor; determining a vibration signature for the motorfrom the detected drive current; comparing the vibration signature to astandard; continuing operation upon a first result of the comparison;and aborting system operation and requesting remedial action upon asecond result of the comparison opposite to the first result.
 20. Amethod of motor control, comprising the steps of: detecting a rotaryposition of an output shaft of a gearbox connected to a load; driving aninput shaft of the gearbox with a motor; supplying motive power to themotor; detecting rotary position of the input shaft of the gearbox;controlling the motive power supplied to the motor in conjunction withthe detected rotary positions of the gearbox input shaft and outputshaft to control position of the load; determining a deflectioncharacteristic for the motor from the detected rotary positions of theinput and output of the gearbox; comparing the deflection characteristicto a standard; continuing operation upon a first result of thecomparison; and aborting operation and requesting remedial action upon asecond result of the comparison opposite to the first result.